This invention relates to a subsea well drilling system. More particularly, this invention relates to a well assembly having a rotating seal, a means for measuring and controlling the downhole pressure of drilling mud, and a pump placed near the well for pumping the drilling mud back to the surface.
To remove the load on the main drilling fluid pump while drilling a well, it is known in the art to provide an auxiliary pump for returning the drilling fluid to the surface. In fact, using this type of system may obviate the need for a riser, as shown in U.S. Pat. No. 4,149,603. However, reciprocating pumps are difficult to use in subsea locations, and the abrasiveness of cuttings entrained in the drilling fluid can destroy the internal components of a centrifugal pump.
Excessive hydrostatic pressure in drilling fluids used while drilling a well may damage surrounding formations. One method of alleviating pressure in the column of fluid in the annulus is shown in U.S. Pat. No. 4,091,881. Fluid is drawn out of the upper portion of the annulus in the riser, and an inert gas is pumped into the fluid to reduce the density of the fluid and lift the fluid through a separate conduit to the surface. The rate of gas injection is controlled to alter the hydrostatic pressure in the well.
An apparatus and method are provided for controlling hydrostatic pressure in drilling fluid in a subsea well. A main pump forces drilling fluid down the bore of a drill pipe running through a wellhead assembly. The drilling fluid exits the drill pipe at the drill bit at the bottom of the well and begins traveling upward in an annulus surrounding the drill pipe.
A rotating drilling head assembly is attached to the upper end of the wellhead assembly. The drilling head has an inner body rotatably carried in an outer body, and the outer body is landed in a bore of a housing, the bore of the housing aligning with the annulus around the drill pipe. The drill pipe is located within a bore of the inner body, and an energizable gripper located in the bore of the inner body grips an outer surface of the drill pipe. Drilling fluid travels upward in the annulus and into bore of the housing. A lateral passage is formed through a sidewall of the housing and communicates the bore of the housing. The drilling head prevents the drilling fluid from moving further upward in the bore of the housing, diverting the drilling fluid into the lateral passage. The drilling head also isolates from the well hydrostatic fluid pressure in the riser.
A submersible pump forces seawater into a conduit extending from the outlet of the pump to the surface for delivering seawater to a surface vessel. A venturi in the conduit creates a lower pressure area due to the seawater flowing through the venturi. The lateral passage of the housing is connected to the venturi, and drilling fluid is drawn out of the annulus and into the conduit. The drilling fluid mixes with the seawater to be carried back to the surface vessel.
Drilling fluid pressure and flow rate measurements are sent to a control system. The control system modulates the rate of operation of each pump for increasing or decreasing hydrostatic pressure in the fluid in the well.